Richard Dormer took over the role of Beric Dondarrion from David Michael Scott in Game of Thrones season 3 - but what was the reason for the recast? The head of House Dondarrion was a minor character in the first season before he remerged in Game of Thrones season 3 as a member of the Brotherhood. In total, Beric appeared in a total of 14 episodes within the entire series.
Beric was the Lord of Blackhaven when he made his Game of Thrones debut. When Ned Stark was still Hand of the King, he sent Beric to apprehend and execute Ser Gregor Clegane for the crimes he committed in the Riverlands. Beric was killed in the conflict but was then resurrected by the Red Priest, Thoros of Myr. Beric and his men then gathered others and formed the Brotherhood Without Banners, a group that followed the Red God. The guerrilla group was imperative to Jon Snow's efforts beyond the Wall, as well as with the Battle of Winterfell.
David Michael Scott's played Beric only once in Game of Thrones season 1, episode 6, "A Golden Crown", where he was listed as a featured extra. When Beric reappeared in season 3, it was with the face of Richard Dormer (no relation to Margaery Tyrell actor Natalie Dormer). And even though Scott had prior acting experience, most notably with his time on The Tudors, his time on Game of Thrones was always intended to be short-lived.
With the Lightning Lord having a much more expanded role starting in Game of Thrones season 3, the team behind the series most likely took their time casting the part for the long-term and settled on Dormer. As he'd only appeared in one scene two years previous, the showrunners likely assumed viewers wouldn't remember Scott or notice the switch.
While Beric was only a small part of season 3, establishing his many resurrections, Dormer's casting was likely made knowing he'd return down the line. The actor reprised his role in Game of Thrones season 6 where the Brotherhood reunited with the Hound, before travelling North and eventually teaming up with Jon Snow to help him capture a wight.
Thoros was killed in Game of Thrones season 7, meaning that Beric was on his last life. That didn't stop him from fighting against the Night King and the army of the dead in Winterfell. Beric, the Hound, and their men joined Jon and Daenerys' forces in defending the living. While inside the walls of the castle, Beric found Arya being chased by a horde of wights. He was killed after bravely sacrificing himself to save the young Stark. With that, Beric Dondarrion became a hero in the eyes of the Game of Thrones audience and Richard Dormer will always be remembered for portraying the character.